U.S. patent application number 10/367098 was filed with the patent office on 2004-02-19 for electrophoretic display with dual mode switching.
Invention is credited to Chung, Jerry, Liang, Rong-Chang.
Application Number | 20040032389 10/367098 |
Document ID | / |
Family ID | 32907618 |
Filed Date | 2004-02-19 |
United States Patent
Application |
20040032389 |
Kind Code |
A1 |
Liang, Rong-Chang ; et
al. |
February 19, 2004 |
Electrophoretic display with dual mode switching
Abstract
The present invention relates to an improved EPD which comprises
both the traditional up/down switching and the in-plane switching
modes. In other words, the improved EPD has dual switching modes.
The monochrome EPDs of the present invention are capable of
displaying highlight color of choice which is different from the
text. For example, white background, blue text, and red highlight
can be shown in any selected areas of the display. Furthermore, the
full color EPDs of the present invention are capable of displaying
high contrast images of high color saturation. Both high quality
black and white states are possible in the full color displays of
the present invention. The EPDs of the present invention do not
need complex circuitry design, and are compatible with low cost and
high yield roll-to-roll manufacturing processes. The EPD cells of
the present invention may have opaque partition walls, or a black
matrix top surface of the partition walls or a combination
thereof.
Inventors: |
Liang, Rong-Chang;
(Cupertino, CA) ; Chung, Jerry; (Mountain View,
CA) |
Correspondence
Address: |
HELLER EHRMAN WHITE & MCAULIFFE LLP
275 MIDDLEFIELD ROAD
MENLO PARK
CA
94025-3506
US
|
Family ID: |
32907618 |
Appl. No.: |
10/367098 |
Filed: |
February 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10367098 |
Feb 14, 2003 |
|
|
|
10222036 |
Aug 16, 2002 |
|
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Current U.S.
Class: |
345/107 |
Current CPC
Class: |
G09G 2300/0426 20130101;
G09G 2310/0262 20130101; G09G 3/344 20130101; G09G 2300/08
20130101; G09G 2300/06 20130101; G02F 1/1677 20190101; G02F 1/16762
20190101; G09G 2320/0252 20130101; G02F 1/1681 20190101; G02F
1/134363 20130101; G09G 2320/02 20130101; G02F 1/167 20130101 |
Class at
Publication: |
345/107 |
International
Class: |
G09G 003/34 |
Claims
What is claimed is:
1. An electrophoretic display comprising a) a top layer comprising
a top electrode, b) a bottom layer comprising a bottom electrode
and at least one in-plane electrode, and c) an array of
electrophoretic cells sandwiched between the top and bottom layers
and each cell comprising (i), opaque partition walls, (ii) a cell
background, (iii) an electrophoretic composition filled therein and
comprises only one type of charged pigment particles dispersed in a
dyed dielectric solvent or solvent mixture.
2. The electrophoretic display of claim 1 wherein said opaque
partition walls are white.
3. The electrophoretic display of claim 1 wherein said opaque
partition walls are gray.
4. The electrophoretic display of claim 1 wherein said opaque
partition walls are formed from a composition comprising a UV
curable material and a filler, dye or pigment.
5. The electrophoretic display of claim 4 wherein said filler is
selected from a group consisting of silica, ZnO, TiO.sub.2,
BaSO.sub.4, CaCO.sub.3 and polymer particles.
6. The electrophoretic display of claim 4 wherein said filler
material is silica or polymer particles.
7. The electrophoretic display of claim 4 wherein said filler
material is in the amount of 1-20% by weight of the
composition.
8. The electrophoretic display of claim 7 wherein said filler
material is in the amount of 2-10% by weight of the
composition.
9. The electrophoretic display of claim 1 wherein said opaque
partition walls are formed from a composition comprising a
radiation curable material and a latent light scattering
material.
10. The electrophoretic display of claim 9 wherein said latent
light scattering material is a material that is compatible with the
radiation curable material before curing but becomes incompatible
after curing.
11. The electrophoretic display of claim 9 wherein said latent
light scattering material is a thermally or photochemically
triggered gas-releasing material.
12. The electrophoretic display of claim 11 wherein said thermally
or photochemically triggered gas-releasing material is selected
from a group consisting of CO.sub.2 producing carboxylic acids,
bicyclic lactones or heterocycles; CO producing ketones or bicyclic
adducts; SO and SO.sub.2 producing sulfones, sulfonyloxy compounds
or heterocycles; N.sub.2 producing azoalkanes, azides,
diazomethanes, N-nitroso compounds, diazo, triazo or tetraazo
heterocycles; COS producing xanthates and O.sub.2 producing
endoperoxides.
13. The electrophoretic display of claim 11 wherein said thermally
or photochemically triggered gas-releasing material is a phenol,
tropolone, pyridine, pyrazine, pyrrole or a halogenated derivative
thereof.
14. The electrophoretic display of claim 1 wherein said opaque
partition walls are colored.
15. The electrophoretic display of claim 1 wherein the inside
surface of the partition walls is of the same color as that of the
electrophoretic composition.
16. The electrophoretic display of claim 15 wherein the two sides
of a partition wall are of different colors.
17. An electrophoretic display comprising a) a top layer comprising
a top electrode and optionally a top substrate, b) a bottom layer
comprising a bottom-electrode, at least one in-plane electrode and
optionally a bottom substrate, and c) an array of electrophoretic
cells sandwiched between the top and bottom layers and each cell
comprising (i) partition walls, (ii) a black matrix layer on the
top surface of the partition walls or on another layer in areas
corresponding to the top surface of the partition walls, (iii) a
cell background, and (iv) an electrophoretic composition filled
therein and comprises only one type of charged pigment particles
dispersed in a dyed dielectric solvent or solvent mixture.
18. The electrophoretic display of claim 17 wherein said black
matrix layer is on the top surface of the partition walls.
19. The electrophoretic display of claim 17 wherein each of said
electrophoretic cells comprises a top sealing layer enclosing said
electrophoretic composition within each cell.
20. The electrophoretic display of claim 19 wherein said top
electrode layer is applied with an adhesive.
21. The electrophoretic display of claim 20 wherein said black
matrix layer is between the top-sealing layer and the adhesive
layer.
22. The electrophoretic display of claim 20 wherein said black
matrix layer is between the adhesive layer and the top electrode
layer.
23. The electrophoretic display of claim 17 wherein said black
matrix layer is between the top electrode layer and the top
substrate.
24. The electrophoretic display of claim 17 wherein said black
matrix layer is on top of the top substrate.
25. The electrophoretic display of claim 17 wherein said black
matrix layer is between the electrophoretic cells and the bottom
electrode layer.
26. The electrophoretic display of claim 17 wherein said black
matrix layer is between the bottom electrode layer and the bottom
substrate.
27. The electrophoretic display of claim 17 wherein said black
matrix layer is on the bottom surface of the bottom substrate.
28. The electrophoretic display of claim 17 wherein said black
matrix layer is applied with registration to top surface of the
partition walls.
29. The electrophoretic display of claim 17 wherein said black
matrix layer is applied by printing, stamping or
photolithography.
30. The electrophoretic display of claim 17 wherein said black
matrix layer is applied by vapor deposition or sputtering with a
shadow mask.
31. The electrophoretic display of claim 17 wherein the optical
density of said black matrix layer is higher than 0.5.
32. The electrophoretic display of claim 31 wherein the optical
density of said black matrix layer is higher than 1.
33. The electrophoretic display of claim 17 wherein the thickness
of the black matrix layer is in the range of from 0.005 .mu.m to 5
.mu.m.
34. The electrophoretic display of claim 33 wherein the thickness
of the black matrix layer is in the range of from 0.01 .mu.m to 2
.mu.m.
35. The electrophoretic display of claim 17 further comprising a
diffuser layer directly or indirectly above the black matrix
layer.
36. The electrophoretic display of claim 17 wherein said black
matrix layer is a highly crosslinked black matrix layer.
37. The electrophoretic display of claim 1 or 17 wherein said
charged pigment particles are pigment-containing
microparticles.
38. The electrophoretic display of claim 37 wherein said
pigment-containing microparticles have a specific gravity matched
to that of the dielectric solvent.
39. The electrophoretic display of claim 17 wherein said partition
walls are opaque.
40. The electrophoretic display of claim 39 wherein said partition
walls are white opaque.
41. The electrophoretic display of claim 39 wherein said partition
walls are gray opaque.
42. The electrophoretic display of claim 17 wherein said partition
walls are colored.
43. The electrophoretic display of claim 17 wherein the inside
surface of the partition walls is of the same color as the
electrophoretic composition.
44. The electrophoretic display of claim 43 wherein the two sides
of a partition wall are of different colors.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation-in-part application of U.S. patent
application Ser. No. 10/222,036 filed on Aug. 16, 2002, the content
of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The electrophoretic display (EPD) is a non-emissive device
based on the electrophoresis phenomenon influencing charged pigment
particles suspended in a solvent. This general type of display was
first proposed in 1969. An EPD typically comprises a pair of
opposed, spaced-apart and plate-like electrodes, with spacers
predetermining a certain distance between the electrodes. At least
one of the electrodes, typically on the viewing side, is
transparent. For the passive type of EPDs, row and column
electrodes on the top (the viewing side) and bottom plates
respectively are needed to drive the displays. In contrast, an
array of thin film transistors (TFT) on the bottom plate and a
non-patterned transparent conductor plate on the top viewing
substrate are required for the active matrix type EPDs. An
electrophoretic fluid composed of a colored dielectric solvent and
charged pigment particles dispersed therein is enclosed between the
two electrodes.
[0003] When a voltage difference is imposed between the two
electrodes, the pigment particles migrate by attraction to the
plate of polarity opposite that of the pigment particles. Thus, the
color showing at the transparent plate may be determined by
selectively charging the plates to be either the color of the
solvent or the color of the pigment particles. Reversal of plate
polarity will cause the particles to migrate back to the opposite
plate, thereby reversing the color. Intermediate color density (or
shades of gray) due to an intermediate level of pigment particles
attracted to the transparent plate may be obtained by controlling
the plate charge through a range of voltages. No backlight is
needed in this type of reflective EPD display, although it may be
optionally added to improve the display viewability in the
dark.
[0004] EPDs of different pixel or cell structures have been
reported previously, for example, the partition-type EPD (see M. A.
Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152
(1979)) and the microencapsulated EPD (as described in U.S. Pat.
No. 5,961,804 and U.S. Pat. No. 5,930,026). Each of these has its
own problems as noted below.
[0005] In a partition-type EPD, there are partitions between the
two electrodes for dividing the space into smaller cells in order
to prevent undesired movements of the particles such as
sedimentation. However, difficulties are encountered in the
formation of the partitions, the process of filling the display
with the fluid, enclosing the fluid in the display, and keeping the
suspensions of different colors separated from each other. Even
more difficult problems are encountered in the development of a
roll-to roll manufacturing process for such a partition type of
displays.
[0006] The microencapsulated EPD has a substantially two
dimensional arrangement of microcapsules each having therein an
electrophoretic composition of a dielectric fluid and a dispersion
of charged pigment particles that visually contrast with the
dielectric solvent. The microcapsules are typically prepared in an
aqueous solution, and to achieve a useful contrast ratio, their
mean particle size is relatively large (50-150 microns). The large
microcapsule size results in a poor scratch resistance and a slow
response time for a given voltage because a large gap between the
two opposite electrodes is required for large capsules. Also, the
hydrophilic shell of microcapsules prepared in an aqueous solution
typically results in sensitivity to high moisture and temperature
conditions. If the microcapsules are embedded in a large quantity
of a polymer matrix to obviate these shortcomings, the use of the
matrix results in an even slower response time and/or a lower
contrast ratio. To improve the switching rate, a charge-controlling
agent is often needed in this type of EPDs. However, the
microencapsulation process in aqueous solution imposes a limitation
on the type of charge-controlling agents that can be used. Other
drawbacks associated with the microcapsule system include poor
resolution and poor addressability for color applications.
[0007] U.S. Pat. No. 3,612,758 discloses another type of EPDs
wherein the electrophoretic cells are formed from parallel line
reservoirs containing charged pigment particles dispersed in a
dielectric solvent. The channel-like electrophoretic cells are
covered with, and in electric contact with, transparent conductors.
A layer of transparent glass from which side the panel is viewed
overlies the transparent conductors.
[0008] The use of microchannels, microgrooves or microcolumns to
form the EPD array still has problem of undesirable particle
sedimentation or creaming along the column direction. In addition,
the lack of a seamless, air-pocket free and continuous sealing
process to enclose the electrophoretic fluid in between the two
electrodes makes the roll-to-roll manufacturing extremely
difficult.
[0009] An improved EPD technology and a roll-to-roll manufacturing
process were recently disclosed in co-pending applications, U.S.
Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. Ser. No.
09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed
on Jun. 28, 2000 and U.S. Ser. No. 09/784,972, filed on Feb. 15,
2001, all of which are incorporated herein by reference. The
improved EPD comprises closed cells formed from microcups of
well-defined shape, size and aspect ratio and filled with charged
pigment particles dispersed in a dielectric solvent and top-sealed
with a top-sealing composition by one of the methods as disclosed
in U.S. Ser. No. 09/518,488 (corresponding to WO 01/67170) and U.S.
Ser. No. 09/874,391 (corresponding to WO02/98977), the contents of
which are incorporated herein by reference. This improved
technology involving microcups allows high image quality in
monochrome EPDs. A color display may also be manufactured by using
a spatially adjacent array of small pixels formed of microcups
filled with dyes of different colors (e.g., red, green or blue).
However, there is a major deficiency in this type of system with
only the traditional up/down switching mode, that is, the white
light reflected from the "turned-off" colored pixels greatly
reduces the color saturation of the "turned-on" color. More details
in this regard are given in the following "Detailed Description"
section.
[0010] While this latter deficiency may be remedied by an overlaid
shutter device such as a polymer dispersed liquid crystal to switch
the "off" pixels to the black state and keep the "on" pixels in the
desired color state, the disadvantage of this approach is the high
cost of the overlaid device, the high Dmin (which is the minimum
optical density of the background) associated with the shutter
device, and the complicated driving circuit design.
[0011] Alternatively, color displays of the normal up/down
switching mode may be prepared by using color filters overlaid on
the viewing side of the display. However, dark Dmin and lack of a
high quality "white" state are the major problems for reflective
color displays using color filters.
[0012] The "in-plane" switching concept was disclosed in E. Kishi,
et al., "5.1: development of In-Plane EPD", Canon Research Center,
SID 00 Digest, pages 24-27 and Sally A. Swanson, et al., "5.2: High
Performance EPDs", IBM Almaden Research Center, SID 00 Digest,
pages 29-31. In these prior art systems, the color effect is
achieved by using a colored background with white or black
particles. The disadvantage of these prior art systems is the lack
of either high quality "white" or high quality "black" state. More
details are also given in the "Detailed Description" section.
[0013] Thus, there is still a need for an improved EPD with high
quality full color capability that can also be manufactured in an
efficient manner, particularly by a roll-to-roll manufacturing
process.
SUMMARY OF THE INVENTION
[0014] The present invention relates to an improved EPD which
comprises both the traditional up/down switching and the in-plane
switching modes. In other words, the improved EPD has dual
switching modes.
[0015] The monochrome EPDs of the present invention are capable of
displaying highlight color of choice which is different from the
color of the text. For example, white background, blue text, and
red highlight can be shown in any selected areas of the display.
Furthermore, the full color EPDs of the present invention are
capable of displaying high contrast images of high color
saturation. Both high quality black and white states are possible
in the full color displays of the present invention. The EPDs of
the present invention do not need complex circuitry design, and are
compatible with low cost and high yield roll-to-roll manufacturing
processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] It is noted that all figures are shown as schematic and are
not to scale.
[0017] FIG. 1 illustrates the common deficiency of the traditional
EPDs with only the up/down switching mode.
[0018] FIGS. 2A-2D illustrate the lack of true white or true black
state in an EPD with only the in-plane switching mode.
[0019] FIG. 3 illustrates a typical electrophoretic cell of the
present invention and the general locations of the up/down and
in-plane switching electrodes.
[0020] FIGS. 4A-4C illustrate the various possible scenarios of the
improved EPD with dual modes.
[0021] FIG. 4D illustrates the highlight option of the present
invention (top view).
[0022] FIGS. 5A and 5B illustrate the manufacture of microcups
involving imagewise photolithographic exposure through
photomask.
[0023] FIGS. 6A and 6B illustrate the two-layered ITO electrode
system.
[0024] FIGS. 7A and 7B illustrate the true white state of the
present invention, the top and cross-section views.
[0025] FIGS. 8A and 8B illustrate the true black state of the
present invention, the top and cross-section views.
[0026] FIGS. 9A and 9B illustrate the multiple color state of the
present invention, the top and cross-section views.
[0027] FIGS. 10A-10E illustrate the TFT active driving
mechanism.
[0028] FIGS. 11A-11E illustrate the combination of the active and
passive driving mechanisms.
[0029] FIGS. 12A-12E illustrate an alternative combination of the
active and passive driving mechanisms.
[0030] FIGS. 13A and 13B illustrate microcups having transparent
and opaque partition walls, respectively. FIG. 13C illustrates the
two sides of a partition wall having different colors.
[0031] FIGS. 14A-14H illustrate the cross-section views of various
black matrix layer locations. FIG. 14I illustrates the top view of
microcups having partition walls with a black matrix layer.
DETAILED DESCRIPTION OF THE INVENTION
[0032] Unless defined otherwise in this specification, all
technical terms are used herein according to their conventional
definitions as they are commonly used and understood by those of
ordinary skill in the art. The terms "well-defined", "aspect ratio"
and "imagewise exposure" are as defined in the copending
applications identified above.
[0033] It is understood that the scope of the present invention
encompasses the conventional EPDs and EPDs manufactured from
microcups, microchannels, microcapsules and the like.
[0034] The term "conventional EPD" refers to any electrophoretic
cells known in the art. The electrophoretic cells may be of any
shapes and sizes, and the displays include, for example, the
partition type displays.
[0035] The term "microchannel" refers to the type of
electrophoretic cells disclosed in U.S. Pat. No. 3,612,758.
[0036] The term "microcup" refers to the cup-like indentations,
which may be created by methods such as microembossing or imagewise
exposure followed by a development step to remove the unexposed or
exposed areas. Likewise, the plural form "microcups" in a
collective context may in general refers to the microcup assembly
comprising a plurality of such microcups integrally formed or
joined to make a structured two-dimensional microcup array. The
dimensions of the microcup are disclosed in the co-pending
applications identified above.
[0037] The term "top-sealing" is intended to refer to a sealing
process in which the display cells constructed on a first substrate
or electrode layer are filled and top-sealed. In the conventional
edge seal process, two substrates or electrode layers and an edge
seal adhesive are required to enclose and edge-seal the display
fluid in the cell(s). In contrast, in the top-sealing process, the
display fluid is enclosed and top-sealed before a second substrate
or electrode layer is disposed onto the display cell(s).
[0038] The term "Dmax" refers to the maximum achievable optical
density of the display.
[0039] The term "Dmin" refers to the minimum optical density of the
display background.
[0040] The term "contrast ratio" is defined as the ratio of the %
reflectance of an electrophoretic display at the Dmin state to the
% reflectance of the display at the Dmax state.
[0041] I. The Disadvantages of Conventional EPDs
[0042] (1) EPD with Only Up/Down Switching
[0043] The EPD of FIG. 1 has only the up/down switching mode. The
cells in the figure are filled with a suspension in which white
positively charged particles are dispersed in a colored (red, green
and blue) dielectric fluid. All three cells in FIG. 1 are shown
charged with a voltage difference between the top and bottom
electrodes (not shown). In the green and blue cells, the top
electrode has a low voltage, the white positively charged particles
in these two cells migrate to the top viewing electrode which is
transparent, and as a result, the color of the particles (i.e.,
white) is reflected to the viewer through the transparent conductor
film in the two cells. In the red cell, the bottom electrode has a
low voltage; consequently the white positively charged particles
migrate to the bottom of the cell, and the color of the medium
(i.e., red) is seen through the top transparent conductor film. In
the scenario as shown in FIG. 1, the white light reflected from the
green and blue pixels greatly reduces the color saturation of the
red pixel.
[0044] (2) EPD with Only the In-Plane Switching Mode
[0045] FIGS. 2A-2D illustrates the disadvantages of the prior art
EPDs with only the in-plane switching mode.
[0046] In FIG. 2A, the cells are filled with a colorless dielectric
solvent with white charged particles dispersed therein. The
background of the cells is colored (i.e., red, green or blue). When
there is a voltage difference between the in-plane electrodes, the
white particles migrate to either side of the cell, and the color
of the background (i.e., red, green or blue) is seen from the top
transparent opening. When there is no voltage difference between
the in-plane electrodes, the particles are scattered in the
dielectric solvent, resulting in white color (i.e., the color of
the particles) being seen from the top transparent opening. This
arrangement of colorless solvent, colored background and white
particles results in a display lack of a high density black
state.
[0047] In FIG. 2B, the cells are filled with a colorless fluid with
black particles dispersed therein. The background of the cells is
colored (i.e., red, green or blue). When there is a voltage
difference between the in-plane electrodes, the particles migrate
to either side of the cell, and the color of the background (i.e.,
red, green or blue) is seen from the top transparent opening. When
there is no voltage difference between the in-plane electrodes, the
particles are scattered in the dielectric solvent, resulting in a
black color (i.e., the color of the particles) being seen from the
top transparent opening. This arrangement of
solvent/background/particle colors results in a dirty white state
with undesirable Dmin and contrast ratio.
[0048] FIG. 2C shows the cells filled with a colorless fluid with
colored particles (i.e., red, green or blue) dispersed therein. The
background of the cells is black. When there is a voltage
difference between the in-plane electrodes, the colored charged
particles migrate to either side of the cell, and the color of the
background (i.e., black) is seen from the top transparent opening.
When there is no voltage difference between the in-plane
electrodes, the colored particles are scattered in the dielectric
solvent, resulting in the color of the particles (i.e., red, green
or blue) being seen from the top transparent opening. In this
design, the black state is of high quality. However, no high
quality white state is possible. As the result, the reflective
display of this type appears to have a dirty background or a low
degree of reflection in the Dmin area.
[0049] In FIG. 2D, the cells are filled with a colorless fluid with
colored particles (red, green or blue) dispersed therein. The
background of the cells is white. When there is a voltage
difference between the in-plane electrodes, the particles migrate
to either side of the cell, and the color of the background (i.e.,
white) is seen from the top transparent opening, resulting in a
high quality white state. When there is no voltage difference
between the in-plane electrodes, the particles are scattered in the
fluid, resulting in the color of the particles (i.e., red, green or
blue) being seen from the top transparent opening. No high quality
black state is available in this design.
[0050] In summary, the in-plane only switching mode results in
either a reflective color display having no high quality black
state or a display having no high quality white state. Contrast
ratio and color saturation are poor in this type of in-plane
switching, reflective color displays. In all in-plane switching
EPDs, the substrate on the opposite side of the in-plane electrodes
is typically a transparent insulator, which usually is the viewing
side of the display.
[0051] II. Electrophoretic Display of the Present Invention
[0052] FIG. 3 illustrates a side view of a typical electrophoretic
cell of the present invention. While only a cup-like cell is
depicted, it is understood that the scope of the present invention
encompasses cells formed from microchannels and the like, and all
types of conventional electrophoretic cells.
[0053] The cell (30) is sandwiched between a top (31) and a bottom
layer (32). The top layer contains a transparent top electrode (not
shown). The bottom layer (32) consists of a layer (32a) comprising
an in-plane switching electrode (34) on the left-hand side, a
bottom electrode (35) and another in-plane electrode (36) on the
right-hand side and optionally a colored background layer (32b).
There is a gap (37) to separate the two in-plane electrodes (34,
36) from the bottom electrode (35).
[0054] The background layer (32b) may be on top of the electrode
layer (32a) (not shown), or underneath the electrode layer (32a).
Alternatively, the layer 32a may serve as the background layer and
in this case, the layer 32a may be black or of other colors.
[0055] Another alternative is that the bottom layer may have only
one in-plane switching electrode, and one bottom electrode with a
gap in between.
[0056] Typically, the cells in FIG. 3 are filled with a clear, but
colored (i.e., red, green or blue) dielectric solvent (38) with
white particles (39) dispersed therein, and the background color of
the cells is typically black. The particles may be positively or
negatively charged. For the purpose of illustration, it is assumed
that the particles are positively charged throughout this
application.
[0057] The charged particles in the individual cells of a display
may be of the same color or of different colors. The individual
cells may also be filled with an electrophoretic fluid containing
charged particles of mixed colors.
[0058] As shown in FIGS. 4A-4C, the dual switching mode allows the
particles to move in either the vertical (up/down) direction or the
planar (left/right) direction. For example, in FIG. 4A, the voltage
of the top electrode is set low, and the voltages of the bottom
electrode and the in-plane electrodes are set high. The white
particles migrate to and gather at the top transparent conductor
film, and the white color (i.e., the color of the particles) is
seen by the viewer.
[0059] In FIG. 4B, the in-plane electrodes are set at low voltages,
and the top and the bottom electrodes are set at high voltages. In
this scenario, the white particles migrate to the sides of the
cells, the color seen through the top transparent conductor film
therefore is the color of the background (i.e., black).
[0060] In FIG. 4C, when the voltage of the top electrode is set
high, the voltage of the bottom electrode is set low, and the
in-plane electrodes are set at low voltage, the white particles
migrate to the bottom of the cells. In this scenario, the color of
the fluid (i.e., red, green or blue) is seen by the viewer through
the top transparent conductor film as shown in the red cell of FIG.
4C. To present a red pixel in a full color display, the white
particles in the green and blue cells may be either attracted to
the sides as shown in FIG. 4C or to the top (not shown). The former
is preferred because it typically exhibits a better color
saturation than the latter. Thus the dual switching mode technology
gives the first full-color EPD wherein all colors including red,
green, blue, black and white of high quality are available in the
same device.
[0061] Furthermore, the background color may be of any color (e.g.,
cyan, yellow or magenta) instead of the commonly used black color.
For example, the cells of FIG. 3 may be filled with a red clear
dielectric solvent with white positively charged particles
dispersed therein and the background color of the cells may be
yellow. In this case, when the particles migrate to the top, the
white color (i.e., the color of the particles) is seen by the
viewer and when the particles migrate to cover the bottom of the
cells, the color of the medium (i.e., red) is seen through the
transparent conductor. However, when the white particles migrate to
the sides of the cells, the color seen through the top transparent
conductor film, will be a shade of orange.
[0062] Other shades or color tones may be achieved by using
different particle/medium/background color combinations, for
example, white/red/cyan, white/red/magenta, white/blue/yellow,
white/blue/cyan, white/blue/magenta, white/green/yellow,
white/green/cyan, white/blue/magenta, etc.
[0063] The preferred combination to achieve a full color display is
white particles, black background, and fluids separately colored
with an additive primary color (i.e., red, green or blue).
[0064] A further aspect of the invention is a monochrome display
with highlight options. In such a case, all cells in the display
have the same background color and are filled with the same
electrophoretic fluid (i.e., having the same particle/solvent color
combination). For example, the display may have white particles,
the solvent is one of the primary colors (red, green or blue) and
the background color is a color contrasting the solvent color. This
arrangement is useful for a relatively simple two color device with
a colored highlight option. For example, an EPD having white
particles, a yellow dielectric solvent, and a black background can
display at least three different colors in each cell or pixel as
shown in FIG. 4D (top view). When the white particles are all
attracted to the top viewing electrode, the white color is seen.
When the white particles are uniformly attracted to the bottom
electrodes, the yellow color is seen through the top viewing
electrode. When the white particles are attracted to the in-plane
electrode on either side of the cell, the black color is seen
through the top viewing electrode. Intermediate colors are also
possible if the particles are driven to intermediate states. This
highly desirable feature of highlight capability in any pixels of
the display can be realized by using the dual switching mechanism
of this invention to drive a low cost monochrome EPD having
pre-selected colors for the particles, the solvent and the
background.
[0065] In summary, the EPDs of the present invention with the dual
switching mode can provide the previously unattainable high quality
full color EPDs and a monochrome EPD with highlight color
capability in any pixels of a monochrome display.
[0066] III. Preparation of Electrophoretic Cells of the Present
Invention
[0067] The microcups generally may be manufactured by
microembossing or photolithography as disclosed in U.S. Ser. No.
09/518,488 filed Mar. 3, 2000 and U.S. Ser. No. 09/784,972 filed on
Feb. 15, 2001.
[0068] While only the cup-like cells are illustrated in the
figures, it is understood that conventional electrophoretic cells
and electrophoretic cells prepared from microchannels, microcolumns
and the like are also within the scope of the present
invention.
[0069] III(a) Preparation of the Microcup Array by Microembossing
Preparation of the Male Mold
[0070] The male mold may be prepared by any appropriate method,
such as a diamond turn process or a photoresist process followed by
either etching or electroplating after the resist is developed. A
master template for the male mold may be manufactured by any
appropriate method, such as electroplating. With electroplating, a
glass base is sputtered with a thin layer (typically 3000 .ANG.) of
a seed metal such as chrome inconel. It is then coated with a layer
of photoresist and exposed to UV. A mask is placed between the UV
and the layer of photoresist. The exposed areas of the photoresist
become hardened. The unexposed areas are then removed by washing
them with an appropriate solvent. The remaining hardened
photoresist is dried and sputtered again with a thin layer of seed
metal. The master is then ready for electroforming. A typical
material used for electroforming is nickel cobalt. Alternatively,
the master can be made of nickel by electroforming or electroless
nickel deposition as described in "Continuous manufacturing of thin
cover sheet optical media", SPIE Proc. 1663:324 (1992). The floor
of the mold is typically between about 50 to 400 microns thick. The
master can also be made using other microengineering techniques
including e-beam writing, dry etching, chemical etching, laser
writing or laser interference as described in "Replication
techniques for micro-optics", SPIE Proc. 3099:76-82 (1997).
Alternatively, the mold can be made by diamond turning or
photomachining using plastics, ceramics or metals.
[0071] The male mold thus prepared typically has protrusions
between about 3 to 500 microns, preferably between about 5 to 100
microns, and most preferably about 10 to 50 microns, and can be of
any shape like round, square, or of other geometry. The male mold
may be in the form of a belt, a roller, or a sheet. For continuous
manufacturing, the belt or the roller type of mold is preferred.
Prior to applying a UV curable resin composition, the mold may be
treated with a mold release to aid in the demolding process. To
further improve the demolding process, the conductor film may be
precoated with a primer or an adhesion promoting layer to improve
the adhesion between the conductor and the microcups.
[0072] Microcups may be formed either in a batchwise process or in
a continuous roll-to-roll process as described in U.S. Ser. No.
09/784,972 filed on Feb. 15, 2001, the content of which is
incorporated herein by reference.
[0073] In the first step of the microembossing process, a UV
curable resin is typically coated on a transparent patterned
conductor film, by any appropriate means, such as roller coating,
die coating, slot coating, slit coating, doctor blade coating, and
the like. The conductor film is usually prepared by sputtering
coating on a plastic substrate such as polyethylene terephthalate,
polyethylene naphthate, polyaramid, polyimide, polycycloolefin,
polysulfone and polycarbonate. The radiation curable material used
is a thermoplastic or thermoset precursor, such as multifunctional
acrylate or methacrylate, vinylether, epoxide and their oligomers,
polymers and the like. Multifunctional acrylates and their
oligomers are the most preferred. A combination of multifunctional
epoxide and multifunctional acrylate is also very useful to achieve
desirable physico-mechanical properties. The UV curable resin may
be degassed prior to dispensing and may optionally contain a
solvent. The solvent, if present, readily evaporates.
[0074] The radiation curable material coated on the conductor
film/substrate is embossed by the male mold under pressure. If the
male mold is metallic and opaque, the conductor film/substrate is
typically transparent to the actinic radiation used to cure the
resin. Conversely, the male mold can be transparent and the
conductor film/substrate can be opaque to the actinic
radiation.
[0075] After exposure to radiation, the radiation curable material
becomes hardened. The male mold is then released from the microcups
formed.
[0076] III(b) Preparation of Microcup Array by Photolithography
[0077] The photolithographic process for preparation of the
microcup array is shown in FIGS. 5A and 5B.
[0078] As shown in FIGS. 5A and 5B, the microcup array may be
prepared by exposure of a radiation curable material (51a), coated
by any known methods onto a transparent patterned conductor film
(52), to UV light (or alternatively other forms of radiation,
electron beams and the like) through a mask (56) to form walls
(51b) corresponding to the image projected through the mask (56).
The conductor film (52) is on a plastic substrate (53).
[0079] In the photomask (56) in FIG. 5A, the dark squares (54)
represent the area opaque to the radiation employed, and the space
(55) between the dark squares represents the radiation-transparent
area. The UV radiates through the opening area (55) onto the
radiation curable material (51a).
[0080] As shown in FIG. 5B, the exposed areas (51b) become hardened
and the unexposed areas (protected by the opaque area (54) of the
mask (56)) are then removed by an appropriate solvent or developer
to form the microcups (57). The solvent or developer is selected
from those commonly used for dissolving or dispersing radiation
curable materials such as methylethylketone, toluene, acetone,
isopropanol or the like.
[0081] Alternatively, the exposure can be done by placing the
photomask underneath the conductor film/substrate. In this case,
the conductor film/substrate must be transparent to the radiation
wavelength used for exposure.
[0082] III(c) Opaque Partition Walls
[0083] In one embodiment, the partition walls of the microcups are
preferably opaque (such as white opaque or gray opaque). FIGS. 13A
and 13B are simplified cross-section views of the microcups having
transparent or opaque partition walls, respectively. FIG. 13A shows
that the electrophoretic composition in one microcup may be seen
from the top of a neighboring microcup through the transparent
partition wall dividing the two microcups. In such a case, more
than one color may be perceived by the viewer which results in a
color shift or parallax (double image). This phenomenon may be
avoided if the partition wall is opaque as shown in FIG. 13B. The
opaque partition walls may be achieved by adding a filler material
such as silica, ZnO, TiO.sub.2, BaSO.sub.4, CaCO.sub.3 or polymer
particles, preferably silica or polymer particles in the amount of
1-20% by weight, preferably in the amount of 2-10% by weight, into
the radiation curable material for the formation of the microcups
as described above.
[0084] The partition walls may also be colored by a dye or pigment
in the radiation curable material for the formation of the
microcups. The filler, dye or pigment used in the radiation curable
composition should not interfere with the polymerization or
crosslinking of the composition during the microcup forming
process. The maximum concentration of the filler, dye or pigment in
the radiation curable microcup formulation is dependent on the
optical density of the composition and the type of radiation used
to cure the composition. To facilitate the microcup formation and
mold release, the resultant radiation curable composition must
allow enough light or radiation to reach the bottom of the
microcups to assure proper physicomechanical properties of the
microcups and their adhesion to the substrate underneath.
[0085] A latent light scattering material may be used to produce
the opaque partition walls. Light scattering centers such as air
pockets or dispersed polymer phase may be produced from the latent
material during or after the microcup forming step(s). For example,
a filler material that is marginally compatible with the radiation
curable composition but incompatible with the cured microcups may
be added in the radiation curable composition. Upon exposure to
radiation, the filler material phase-separates and forms discrete
light-scattering domains in the cured microcup structure.
Similarly, air pockets may be formed during or after the microcup
forming step(s) by incorporating, into the radiation curable
microcup composition, a thermally or photochemically triggered
gas-releasing material as described in G. J. Sabongi, "Chemical
Triggering, Reactions of Potentially Utility in Industrial
Processes", Plenum Press, (1987). Examples of the gas releasing
reactions may include CO.sub.2 from carboxylic acids, bicyclic
lactones or heterocycles; CO from ketones or bicyclic adducts; SO
and SO.sub.2 from sulfones, sulfonyloxy compounds or heterocycles,
particularly 3-membered and 5-membered heterocycles; N.sub.2 from
azoalkanes, azides, diazomethanes, N-nitroso compounds, diazo,
triazo or tetraazo heterocycles; COS from xanthates; or O.sub.2
from endoperoxides. Air pockets may also be formed by an
ultrasonic-wave triggered gas release reaction of phenol,
tropolone, pyridine, pyrazine, pyrrole or halogenated derivatives
thereof.
[0086] In one embodiment, the inside surface (130) of the partition
walls (131) may be colored to match the color of the dielectric
solvent in the electrophoretic composition as shown in FIG. 13C. In
this scenario, it is possible that the two sides of a partition
wall may have two different colors if the two adjacent
electrophoretic cells are of different colors. For example, if cell
A in FIG. 13C is filled with a red electrophoretic composition
whereas the adjacent cell B is filled with a green electrophoretic
composition, one side (130a) of the partition wall facing the red
cell may be of the red color and the other side (130b) facing the
green cell may be of the green color. The different sides of
partition walls may be colored sequentially by, for example,
impregnating selective microcups in a dye or pigment
solution/dispersion before the selective filling/sealing steps of
forming a color display according the process disclosed in
copending U.S. patent application Ser. No. 09/879,408, filed on
Jun. 11, 2001, the content of which is incorporated herein by
reference.
[0087] The openings of the microcups prepared according to the
methods described above may be round, square, rectangular,
hexagonal, or any other shape. The partition area between the
openings is preferably kept small in order to achieve high color
saturation and contrast ratio while maintaining desirable
mechanical properties. Consequently, the honeycomb-shaped opening
is preferred over, for example, the circular opening.
[0088] For reflective electrophoretic displays, the dimension of
each individual microcup may be in the range of about 10.sup.2 to
about 1.times.10.sup.6 .mu.m.sup.2, preferably from about 10.sup.3
to about 1.times.10 .sup.5 .mu.m.sup.2. The depth of the microcups
is in the range of about 5 to about 200 microns, preferably from
about 10 to about 100 microns. The opening to the total area ratio,
total area being defined as that of one cup including walls
measured from wall centers, is in the range of from about 0.05 to
about 0.95, preferably from about 0.4 to about 0.9.
[0089] The colored background layer of the cells may be added by
painting, printing, coating, vapor deposition, sputtering or
laminating a colored layer onto the bottom layer (the non-viewing
side).
[0090] III(d) Preparation of the Suspensions
[0091] The cells are filled with an electrophoretic composition
comprising charged pigment particles dispersed in a colored
dielectric solvent. The electrophoretic composition may optionally
contain additional colorants that do not migrate in the electric
field. The pigment dispersion may be prepared according to methods
well known in the art, such as U.S. Pat. Nos. 6,017,584, 5,914,806,
5,573,711, 5,403,518, 5,380,362, 4,680,103, 4,285,801,
4,093,534,4,071,430, and 3,668,106. See also IEEE Trans. Electron
Devices, ED-24, 827 (1977), and J. Appl. Phys. 49(9):4820
(1978).
[0092] The dielectric solvent preferably has a low viscosity and a
dielectric constant in the range of about 2 to about 30, preferably
about 2 to about 15 for high particle mobility. Examples of
suitable dielectric solvents include hydrocarbons such as
decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty
oils, paraffin oil, aromatic hydrocarbons such as toluene, xylene,
phenylxylylethane, dodecylbenzene and alkylnaphthalene, halogenated
solvents such as, dichlorobenzotrifluoride,
3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene,
dichlorononane, pentachlorobenzene, and perfluoro solvents such as
perfluorodecalin, perfluorotoluene, perfluoroxylene, FC-43, FC-70
and FC-5060 from 3M Company, St. Paul Minn., low molecular weight
halogen containing polymers such as poly(perfluoropropylene oxide)
from TCI America, Portland, Oreg., poly(chlorotrifluoroethylene)
such as Halocarbon Oils from Halocarbon Product Corp., River Edge,
N.J., perfluoropolyalkylethers such as Galden, HT-200, and
Fluorolink from Ausimont or Krytox Oils and Greases K-Fluid Series
from DuPont, Delaware. In one preferred embodiment,
poly(chlorotrifluoroethylene) is used as the dielectric solvent. In
another preferred embodiment, poly(perfluoropropylene oxide) is
used as the dielectric solvent.
[0093] The dielectric solvent may be colored by a contrasting dye
or pigment. Nonionic azo and anthraquinone dyes are particularly
useful. Examples of useful dyes include, but are not limited to:
Oil Red EGN, Sudan Red, Sudan Blue, Oil Blue, Macrolex Blue,
Solvent Blue 35, Pylam Spirit Black and Fast Spirit Black from
Pylam Products Co., Arizona, Sudan Black B from Aldrich,
Thermoplastic Black X-70 from BASF, anthraquinone blue,
anthraquinone yellow 114, anthraquinone reds 111 and 135,
anthraquinone green 28 from Aldrich. Fluorinated dyes are
particularly useful when perfluorinated solvents are used. In case
of a contrasting pigment, the colorant of the medium may also be
dispersed in the dielectric medium and are preferably uncharged. If
the contrasting color pigment particles are charged, they
preferably carry a charge which is opposite from that of the
charged primary color pigment particles. If both the contrasting
color and the primary color pigment particles carry the same
charge, they should have different charge density or different
electrophoretic mobility. The dyes or pigments used in EPDs must be
chemically stable and compatible with other components in the
suspension.
[0094] The charged primary color particles are preferably white,
and may be organic or inorganic pigments, such as TiO.sub.2,
BaSO.sub.4 or ZnO.
[0095] If colored pigment particles are used, they may be selected
from phthalocyanine blue, phthalocyanine green, diarylide yellow,
diarylide AAOT yellow, and quinacridone, azo, rhodamine, perylene
pigment series from Sun Chemical, Hansa yellow G particles from
Kanto Chemical, and Carbon Lampblack from Fisher. Particle size is
preferably in the range of 0.01-5 microns, and is even more
preferably in the range of 0.05-2 microns. The particles should
have acceptable optical characteristics, should not be swollen or
softened by the dielectric solvent, and should be chemically
stable. The resulting suspension must also be stable against
sedimentation, creaming or flocculation under normal operating
conditions.
[0096] The migrating pigment particles may exhibit a native charge,
or may be charged explicitly using a charge control agent, or may
acquire a charge when suspended in the dielectric solvent. Suitable
charge control agents are well known in the art; they may be
polymeric or non-polymeric in nature, and may also be ionic or
non-ionic, including ionic surfactants such as Aerosol OT, sodium
dodecylbenzenesulfonate, metal soaps, polybutene succinimide,
maleic anhydride copolymers, vinylpyridine copolymers,
vinylpyrrolidone copolymer (such as Ganex from International
Specialty Products), (meth)acrylic acid copolymers, and
N,N-dimethylaminoethyl (meth)acrylate copolymers. Fluorosurfactants
are particularly useful as charge controlling agents in
perfluorocarbon solvents. These include FC fluorosurfactants such
as FC-170C, FC-171, FC-176, FC430, FC431 and FC-740 from 3M Company
and Zonyl fluorosurfactants such as Zonyl FSA, FSE, FSN, FSN-100,
FSO, FSO-100, FSD and UR from DuPont.
[0097] Suitable charged pigment dispersions may be manufactured by
any of the well-known methods including grinding, milling,
attriting, microfluidizing, and ultrasonic techniques. For example,
pigment particles in the form of a fine powder are added to the
suspending solvent and the resulting mixture is ball milled or
attrited for several hours to break up the highly agglomerated dry
pigment powder into primary particles. Although less preferred, a
dye or pigment for generating color of the dielectric solvent may
be added to the suspension during the ball milling process.
[0098] Sedimentation or creaming of the pigment particles may be
eliminated by microencapsulating the particles with suitable
polymers to match the specific gravity to that of the dielectric
solvent. Microencapsulation or coating of the pigment particles may
be accomplished chemically or physically. Typical
microencapsulation processes include interfacial polymerization,
in-situ polymerization, phase separation, coacervation,
electrostatic coating, spray drying, fluidized bed coating and
solvent evaporation.
[0099] Density matched pigment-containing microparticles may be
prepared according to methods disclosed in copending U.S. patent
applications, U.S. Serial No. 60/345,936, filed on Jan. 3, 2002,
U.S. Serial No. 60/345,934 also filed on Jan. 3, 2002, U.S. Ser.
No. 10/335,210 filed on Dec. 31, 2002, U.S. Ser. No. 10/335,051
filed on Dec. 31, 2002, U.S. Serial No. 60/400,021 filed on Jul.
30, 2002, U.S. Serial No. 60/418,078 filed on Oct. 10, 2002 and
U.S. Serial No. 60/356,226 filed on Feb. 11, 2002, the contents of
which are incorporated herein by reference in their entirety. The
average particle size of the density-matched pigment-containing
microparticles may be in the range of 0.1-10 .mu.m, preferably in
the range of 0.25-3 .mu.m.
[0100] III(e) Filling and Sealing of the Microcups
[0101] The filling and sealing procedures are described in the
co-pending applications, U.S. Ser. No. 09/518,488, U.S. Ser. No.
09/784,972, U.S. Ser. No. 09/879,408, U.S. Ser. No. 09/874,391 and
U.S. Serial No. 60/408,256, the disclosures of which are
incorporated herein by reference in their entirety.
[0102] After the microcups are filled with an electrophoretic
composition, they are top-sealed. The critical step of top-sealing
of the microcups may be accomplished in a number of ways. A
preferred approach is to disperse a UV curable composition into an
electrophoretic fluid comprising charged pigment or
pigment-containing particles dispersed in a colored dielectric
solvent. The suitable UV curable materials may include acrylates,
methacrylates, styrene, alpha-methylstyrene, butadiene, isoprene,
allylacrylate, polyvalent acrylate or methacrylate, cyanoacrylates,
polyvalent vinyl including vinylbenzene, vinylsilane, vinylether,
polyvalent epoxide, polyvalent isocyanate, polyvalent allyl, and
oligomers or polymers containing crosslinkable functional groups.
The UV curable composition is immiscible with the dielectric
solvent and has a specific gravity lower than that of the
electrophoretic composition comprising pigment particles dispersed
in a dielectric solvent. The two compositions, UV curable
composition and the electrophoretic composition, are thoroughly
blended with, for example, an in-line mixer, and immediately coated
onto the microcups with a precision coating mechanism such as Myrad
bar, gravure, doctor blade, slot coating or slit coating. Excess
fluid is removed by a wiper blade or a similar device. A small
amount of a weak solvent or solvent mixture such as heptane,
isopropanol and methanol may be used to clean the residual
electrophoretic fluid on the top surface of the partition walls of
the microcups. Volatile organic solvents may be used to control the
viscosity and coverage of the electrophoretic fluid. The
thus-filled microcups are then dried and the UV curable composition
floats to the top of the electrophoretic fluid. The microcups may
be top-sealed by curing the supernatant UV curable layer during or
after it floats to the top. The UV light or other forms of
radiation such as visible light, IR or electron beam may be used to
cure the top-sealing layer and seal the microcups. Alternatively,
heat or moisture may also be employed to cure the top-sealing layer
and seal the microcups, if a heat or moisture curable composition
is used.
[0103] A preferred group of dielectric solvents exhibiting
desirable density and solubility discrimination against acrylate
monomers and oligomers are halogenated hydrocarbons, particularly
perfluoro solvents such as perfluoroethers from Ausimont, Italy or
Du Pont, Delaware, and their derivatives. Surfactants may be used
to improve the adhesion and wetting at the interface between the
electrophoretic fluid and the sealing materials. Surfactants
include the FC surfactants from 3M Company, Zonyl fluorosurfactants
from DuPont, fluoroacrylates, fluoromethacrylates,
fluoro-substituted long chain alcohols, perfluoro-substituted long
chain carboxylic acids and their derivatives.
[0104] Alternatively, the electrophoretic fluid and the top-sealing
composition may be applied sequentially onto the microcups to
prevent intermixing, particularly when the sealing precursor is at
least partially compatible with the dielectric solvent. Thus, the
top-sealing of the microcups may be accomplished by overcoating a
thin layer of a top-sealing composition which is hardenable by
radiation, heat, solvent evaporation, moisture or interfacial
reactions on the surface of the filled microcups. Volatile organic
solvents may be used to adjust the viscosity and the thickness of
the coatings. When a volatile solvent is used in the overcoat, it
is preferred that it is immiscible with the dielectric solvent to
reduce the degree of intermixing between the top-sealing layer and
the electrophoretic fluid. To further reduce the degree of
intermixing, it is highly desirable that the specific gravity of
the overcoated top-sealing composition is no greater than that of
the electrophoretic fluid. In the co-pending patent application,
U.S. Ser. No. 09/874,391, filed Jun. 4, 2001, compositions
comprising a thermoplastic elastomer have been disclosed as the
preferred top-sealing composition. In copending patent application
U.S. Serial No. 60/408,256, the content of which is incorporated
herein by reference, compositions comprising a polyurethane have
been disclosed as the preferred top-sealing composition. Additives
such as silica particles, binder polymers or surfactants may be
used to improve the film integrity and coating quality.
[0105] Alternatively, interfacial polymerization followed by UV
curing has been found beneficial to the top-sealing process.
Intermixing between the electrophoretic layer and the top-sealing
overcoat is significantly suppressed by the formation of a thin
barrier layer at the interface by interfacial polymerization or
crosslinking. The top-sealing is then completed by a post curing
step, preferably by UV radiation. The two-step overcoating process
is particularly useful when the dye used is at least partially
soluble in the top-sealing composition.
[0106] III(f) Lamination of the Microcups
[0107] The top-sealed microcups are then laminated with the other
electrode film preferably with an adhesive layer. Suitable adhesive
materials include acrylic and rubber types of pressure sensitive
adhesives, UV curable adhesives containing for example,
multifunctional acrylates, epoxides, vinylethers or thiolene, and
moisture or heat curable adhesives such as epoxy, polyurethane, and
cyanoacrylate.
[0108] In one embodiment of the invention, a substrate containing
thin film transistors may be used as one of the bottom layer
electrodes to also provide the active driving mechanism and the top
electrode, in this scenario, is transparent.
[0109] The second (top) electrode layer may also be disposed on the
top-sealed microcups by coating, printing, vapor deposition,
sputtering or a combination thereof.
[0110] The top electrode layer may also be coated or laminated with
a top substrate layer.
[0111] III(g) Application of a Black Matrix Layer over the Top
Surface of the Partition Walls
[0112] When the viewing side is from the top-sealing side of the
microcups, the black matrix may be applied between the top surface
(140) of the partition walls (141) and the top-sealing layer (142)
as shown in FIG. 14A, between the top-sealing layer (142) and the
adhesive layer (143), if present, as shown in FIG. 14B, between the
adhesive layer (143), if present, and the top electrode layer (144)
as shown in FIG. 14C, between the top electrode layer (144) and the
top substrate (145) as shown in FIG. 14D or on top of the top
substrate (145) as shown in FIG. 14E. When the viewing side is from
the bottom, the black matrix may be applied between the microcups
(146) and the bottom electrode layer (147) as shown in FIG. 14F,
between the bottom electrode layer (147) and the bottom substrate
(148) as shown in FIG. 14G or on the bottom surface of the bottom
substrate (148) as shown in FIG. 14H.
[0113] A black matrix layer may be applied with registration to the
partition walls. In other words, the black matrix layer is on the
top surface of the partition walls or on another layer in areas
corresponding to the top surface of the partition walls. The
openings of the microcups or areas corresponding to the openings of
the microcups are not covered by the black matrix layer. The black
matrix layer may be applied by a method such as printing, stamping,
photolithography, vapor deposition or sputtering with a shadow
mask. The optical density of the black matrix may be higher than
0.5, preferably higher than 1. Depending on the material of the
black matrix and the process used to dispose the black matrix, the
thickness of the black matrix may vary from 0.005 .mu.m to 5 .mu.m,
preferably from 0.01 .mu.m to 2 .mu.m.
[0114] In one embodiment as shown in FIG. 14A, a thin layer of
black coating or ink may be transferred onto the top surface (140)
of the partition walls (141) after formation of the microcups, by
an offset rubber roller or stamp. After the transferred coating or
ink is hardened, the microcups are subsequently filled and
top-sealed (142). In this case, the hardened black matrix must be
resistant to the solvent(s) used in both the electrophoretic and
top-sealing compositions.
[0115] In another embodiment as shown in FIG. 14B, the black matrix
layer may be applied, with registration to the partition walls,
onto the top-sealing layer after the microcups are filled and
top-sealed. For example, a photosensitive black coating may be
coated onto the top-sealing layer (142) and exposed imagewise with
registration through a photomask. The photosensitive black coating
may be a positively-working or negatively-working resist. When a
positively-working resist is used, the photomask should have
openings corresponding to the microcup areas. In this scenario, the
photosensitive black coating in the microcup areas (exposed) is
removed by a developer after exposure. If a negatively-working
resist is used, the photomask should have openings corresponding to
the top surface of the partition walls. In this scenario, the
photosensitive black coating in the microcup areas (unexposed) is
removed by a developer after exposure. The solvent(s) used to apply
the black coating and the developer(s) for removing the coating
should be carefully selected so that they do not attack the
top-sealing layer.
[0116] Alternatively, a colorless photosensitive ink-receptive
layer may be applied onto the top sealing layer followed by
exposure through a photomask. If a positively-working
photosensitive latent ink-receptive layer is used, the photomask
should have openings corresponding to the top surface of the
partition walls. In this scenario, after exposure, the exposed
areas (the microcup areas) become ink-receptive or tacky and a
black matrix may be formed on the exposed areas (the top surface of
the partition walls) after a black ink or toner is applied onto
those areas. Alternatively, a negatively-working photosensitive
ink-receptive layer may be used. In this case, the photomask should
have openings corresponding to the microcups and after exposure,
the exposed areas (the microcup areas) are hardened while a black
matrix layer may be formed on the unexposed areas (the top surface
of the partition walls) after a black ink or toner is applied onto
those areas. The black matrix may be post cured by heat or flood
exposure to improve the film integrity and physicomechanical
properties.
[0117] In another embodiment, the black matrix may be applied by
printing such as screen printing or offset printing, particularly
waterless offset printing.
[0118] In still another embodiment, the black matrix may be applied
onto the adhesive layer, if present, electrode layer or the
substrate layer with registration to the top surface of the
partition walls by any of the methods mentioned above.
[0119] To complete the display assembly, a diffuser layer may be
applied directly or indirectly above the black matrix layer to
improve the visual effect of the finished display device.
[0120] FIG. 14I is the top view of microcups having partition walls
with a black matrix top surface. As shown, the areas over the
openings of the microcups remain transparent. While microcups of a
square opening are shown in FIG. 14I, the top opening shape of the
microcups may vary. It may be rectangular, circular or a more
complex shape if desired.
[0121] The black matrix layer added to the top surface of the color
display significantly improves the contrast ratio and color
saturation of the display. In the case when a highly crosslinked
black matrix layer is formed on the top surface of the display
from, for example, the above-mentioned photosensitive ink-receptive
layer, it may also render the top surface more resistant to scratch
and finger prints.
[0122] While not specifically described, it is understood that the
combination of the black matrix layer as described in this section
and the opaque or colored partition walls as described above in
Section III(c) above is also beneficial.
[0123] IV. The Operations of the Dual Mode Switching
[0124] IV(a) Passive Matrix
[0125] (1) Electrode Circuit Design
[0126] FIG. 6A is the side view of the two-layer passive matrix
electrode circuit design. FIG. 6B shows the top view of a two-layer
passive matrix electrode design for dual-mode. The cells (60) are
sandwiched between one top layer (61) and a bottom layer (62). The
horizontal bars are the row electrodes (63) that are transparent
and run through the top of the cells. The bottom layer (62)
consists of one in-plane electrode (64) that is on the left-hand
side of the cell, one bottom column electrode (65) and another
in-plane electrode (66) on the right-hand side. There are gaps (67)
between the in-plane electrodes and between the in-plane electrodes
and the column electrodes.
[0127] The cross section of the top row electrode, the bottom
column electrode, and the in-plane electrodes define the display
cell.
[0128] (2) Driving Timing Sequences
[0129] To illustrate the dual mode switching of this invention,
electrophoretic cells comprising a common black background and
positively charged white particles dispersed in a clear colored
solvent are used.
[0130] A true white state may be generated by applying the top row
electrodes (63) of selected cells or pixels with a lower voltage
and at the same time applying the bottom column electrodes (65) and
the in-plane electrodes (64) with a higher voltage. At this bias
setting, the particles are driven to the top row electrodes, and
the white color is seen through the top transparent conductor layer
by the viewer.
[0131] FIG. 7A is the cross-sectional view of an array of cells,
which demonstrates the true white state. When the white particles
in all cells migrate to the top of the cells, the resulting color
(i.e., white) is seen from the top through the transparent
conductor film (not shown). FIG. 7B is the top view of the array of
cells showing the true white state.
[0132] A true black state can be achieved by, for example, a
two-step driving process. In the first step, the voltage of the row
electrode (63) is set high and the voltages of the column electrode
(65) and the two in-plane electrodes (64) are set low. As a result,
white particles are first attracted to the bottom of the cells. In
the second step, the voltage of the in-plane electrodes is set low,
the voltage of the column electrode is set high, and the row
electrode is also set high. Under these settings, the white
particles, driven by the electric field, migrate to and cover the
sides of the cells, resulting in the black background color being
seen through the top transparent conductor film.
[0133] Alternatively, a true black state can also be achieved by
using a one-step driving process. More specifically, the black
color may be seen by setting the row (63) and column (65)
electrodes of the selected cells with high voltages, and the
in-plane electrodes (64) at a low voltage. The voltages applied to
the row and column electrodes may not be the same. This allows the
electric field from both the top row electrode (63) and the bottom
column electrode (65) to force the particles in the selected cells
to move quickly towards the edges of the cells and results in a
true black state of high quality.
[0134] FIG. 8A is the cross-sectional view of the same array of
cells (as was shown in FIG. 7A) to demonstrate the true black state
of this invention. The white particles in all cells migrate to the
sides of the cells, resulting in the color of the background (i.e.,
black) being seen from the top transparent conductor film. FIG. 8B
is the top view of the array of cells showing the true black
state.
[0135] A colored (e.g., red, blue, or green) state of the selected
cells can be achieved when the voltage of a top (row) electrode
(63) is set high, and the voltages of a column electrode (65) and
the two in-plane electrodes (64) are set low. In this case, the
white particles in the cell, driven by the electric field, migrate
to the column electrode at the bottom. The bottom of the cells will
be covered by the white particles and the color of the dielectric
solvent (i.e., red, green or blue) is seen through the top
transparent conductor layer. Any color combinations may be achieved
according to the present invention by moving the charged white
particles in selected cells of a pixel to the bottom. Gray scale
can also be obtained by adjusting the voltage to partially move the
particles to the bottom column electrodes.
[0136] FIG. 9A is the cross-sectional view of the same array of
cell (as shown in FIGS. 7A and 8A) to demonstrate the white, black
and two color states in the same EPD display of the present
invention. The cells with the white particles migrated to the top
row electrodes (63) show the white color; the cells with the white
particles migrated to bottom column electrodes (65) show the color
of the dielectric solvent (i.e., red, green or blue); and the cells
with the white particles migrated to the sides of the cells show
the black color. FIG. 9B is the top view of the array of cells
showing multiple colors.
[0137] Although the dual switching mode was illustrated above by
using two in-plane electrodes (64) in a cell, the same results can
also be achieved by using only one in-plane electrode or multiple
in-plane electrodes.
[0138] IV(b) TFT Active Matrix
[0139] (1) Electrode Circuit Design
[0140] TFT (Thick Film Transistor) Active Matrix used in the LCD
display system has been disclosed in U.S. Pat. No. 5,132,820. Such
TFT technology can also be applied to the dual-mode EPD of the
present invention. In a preferred embodiment, the top layer
electrode made of a transparent conductive material, such as ITO,
is in one piece which covers the entire top surface of the display.
The top electrode connects to ground (0V). FIG. 10A shows the top
view of the bottom electrode layer of a 2.times.2 array of cells.
FIG. 10B shows the details of the TFT connection. Each cell (100)
comprises one bottom electrode (101) and two in-plane electrodes
(102). Each of the bottom electrodes connects to the drain (103a)
of a TFT (104a). The in-plane electrodes of each cell connect to
the drain (103b) of another TFT (104b). The sources (105a and 105b)
of the TFTs (104a and 104b) connect to signal lines (106a and 106b)
which run vertically through the bottom surface of the device. The
gates (107a and 107b) of the TFTs (104a and 104b) connect to a scan
line (108), which runs horizontally through the bottom surface of
the device. The scan and signal lines form a matrix structure, but
they are insulated from each other.
[0141] In the dual-mode switching, two TFTs (104a and 104b) are
required for each cell to control the bottom electrode (101) and
the in-plane electrodes (102) independently. When a row of cells
(100) are scanned, the scan line (108) applies a voltage to the
gates (107a and 107b) of the TFTs (104a and 104b) on that row of
cells which turn on the TFTs. At the same time, signals for each
electrode are applied at signal lines (106a and 106b), which are
connected to the sources (105a and 105b) of the TFTs. These signals
are then switched to the drains (103a and 103b) of the TFTs, which
are connected to the bottom electrode and in-plane electrodes
respectively. The signals form the desired bias condition of each
cell. Storage capacitors (109a and 109b) are added to hold the
voltages, so the voltages at the electrodes continue providing the
bias to the cell even after the switching. This driving scheme
speeds up the switching time dramatically. In addition, after the
storage capacitors (109a and 109b) are charged, the driver can
continue switching the next row. The switching time for each row of
electrodes is only the charging time for the storage capacitors.
This greatly reduces the response time of the display.
[0142] (2) Driving Timing Sequences
[0143] To illustrate the dual mode switching of this invention,
electrophoretic cells comprising a common black background and
positively charged white particles dispersed in a clear colored
solvent are used.
[0144] In a preferred embodiment, the top electrode of the device
is permanently connected to ground (0V). As illustrated in FIG.
10C, the scan line is set at a voltage, Von, to turn on all the
TFTs on the scanning row. Voltages at the signal lines are then
switched to the bottom electrode and the in-plane electrodes. When
the bottom electrode and in-plane electrodes are both set at a
negative voltage, particles in the cell move to the bottom surface
of the cell. The bottom of the cell will be covered by the white
particles and the color of the dielectric solvent (i.e., red, green
or blue) is seen through the top transparent conductor layer.
[0145] As illustrated in FIG. 10D, when the bottom electrode and
in-plane electrodes are both set at a positive voltage, particles
in the cell move to the top surface of the cell. When the white
particles in the cell migrate to the top of the cell, the resulting
white color is seen from the top through the conductor layer.
[0146] As illustrated in FIG. 10E, when the bottom electrode is set
at 0V, and the in-plane electrodes are set at a negative voltage,
particles move to the area of the small in-plane electrodes,
resulting in the black background color being seen through the top
transparent conductor film.
[0147] After a row of cells are scanned, the storage capacitors of
each cell are charged according to the signal line voltage. When a
row of cells are not being scanned, it is in the non-scan phase.
During the non-scan phase, the electrode with a voltage set by the
storage capacitor continues to generate the electric field and
control the movement of particles.
[0148] The scan time limit of this design is determined by the
storage capacitor discharge time. The capacitor needs to be
refreshed (recharged) before the voltage drops more than 10% to
maintain a good bias voltage. The display response (on/off) time is
determined by the pixel on/off time, because each pixel can be
quickly charged and refreshed. Thus, the line to line scan delay
can be eliminated.
[0149] IV(c) Combination of Passive Matrix and TFT Active
Matrix
[0150] (1) Electrode Circuit Design
[0151] In this novel design, both the passive matrix electrodes and
the TFT active matrix are used in the driving electrode design.
FIG. 11A shows the top view and FIG. 11B shows the side view of the
driving circuit design. The bottom electrode layer (117) comprises
the bottom column electrode (111) and in-plane electrodes (112).
The in-plane electrodes of each cell is connected to the drain of a
TFT (113). The source of the TFT is connected to a signal line
(114) which runs vertically through the bottom surface of the
device. The gate of the TFT is connected to a scan line (115) which
runs horizontally through the bottom surface of the device. The
scan and signal lines form a matrix structure, but the two types of
lines are insulated from each other. The cells (110) are sandwiched
between one top layer (116) and a bottom layer (117). The
horizontal bars are the top row electrodes (118) that are
transparent and run through the top of the cells (110). There is a
gap (119) separating the two in-plane electrodes (112) and the
column electrode (111). The cross section of the top row electrode,
the bottom column electrode, and the in-plane electrodes define the
display cell.
[0152] The top row electrodes (118) and bottom column electrodes
(111) form the passive matrix which controls the movement of
particles in the up and down direction. The in-plane electrodes
(112) and the bottom column electrode (111) provide the bias
voltage for the in-plane movement of the particles. The in-plane
electrodes are controlled by the TFT active matrix.
[0153] (2) Driving Timing Sequences
[0154] To illustrate the dual mode switching of this invention,
electrophoretic cells comprising a common black background and
positively charged white particles dispersed in a clear colored
solvent are used.
[0155] As illustrated in FIG. 11C, the scan signal is set at a
voltage, Von, to turn on all the TFTs on the scanning row. Voltages
at the signal lines are then switched to the in-plane electrodes.
When the bottom column electrode and in-plane electrodes are both
set at a low voltage and the top row electrode is set at a high
voltage, particles in the cell move to the bottom surface of the
cell. The bottom of the cells consequently is covered by the white
particles and the color of the dielectric solvent (i.e., red, green
or blue) is seen through the top transparent conductor layer.
[0156] As illustrated in FIG. 11D, when the bottom electrode and
in-plane electrodes are both set at a high voltage and the top row
electrode is set at a low voltage, particles in the cell move to
the top surface of the cell. When the white particles in the cell
migrate to the top of the cell, the white color is seen from the
top and the cell is in the white state.
[0157] As illustrated in FIG. 11E, when the top row electrode and
bottom column electrode are set at a high voltage and the in-plane
electrodes are set at a low voltage, particles move to the area of
the small in-plane electrodes, resulting in the black background
color being seen through the top transparent conductor film.
[0158] This design has the advantage of reducing the in-plane
switching time. As the in-plane electrodes are controlled by the
TFTs, the storage capacitor holds the voltage on the in-plane
electrodes. During the non-scan phase, the electrode with a voltage
set by the storage capacitor continues to generate the electric
field and control the movement of particles. This effectively
reduces the cell switching time.
[0159] IV(d) Alternative Combination of Passive Matrix and TFT
Active Matrix
[0160] (1) Electrode Circuit Design
[0161] In another novel design, both the passive matrix electrodes
and TFT active matrix are used in the driving electrode design.
FIG. 12A shows the top view and FIG. 12B shows the side view of the
driving circuit design. The bottom electrode layer (127) comprises
bottom column electrode (121) and in-plane electrode (122). The
bottom electrode of each cell connects to the drain of a TFT (123).
The source of the TFT connects to a signal line (124), which runs
vertically through the bottom surface of the device. The gate of
the TFT connects to a scan line (125), which runs horizontally
through the bottom surface of the device. The scan and signal lines
form a matrix structure, but they are insulated from each other.
The cells (120) are sandwiched between a top layer (126) and a
bottom layer (127). The horizontal bars are the top row electrodes
(128) that are transparent and run through the top of the cells.
There is a gap (129) separating the two in-plane electrodes (122)
and the bottom electrode (121). The cross section of the top row
electrode, the bottom electrode, and the in-plane electrodes define
the display cell.
[0162] The top row electrodes and the in-plane electrodes form the
passive matrix. The in-plane electrodes and the bottom electrode
provide the bias voltage for the in-plane movement of the
particles. The bottom electrodes are controlled by the TFT active
matrix.
[0163] (2) Driving Timing Sequences
[0164] To illustrate the dual mode switching of this invention,
electrophoretic cells comprising a common black background and
positively charged white particles dispersed in a clear colored
solvent are used.
[0165] As illustrated in FIG. 12C, the scan signal is set at a
voltage, Von, to turn on all the TFTs on the scanning row. Voltages
at the signal lines are then switched to the bottom electrode. When
the bottom electrode and in-plane electrodes are both set at a low
voltage, and the top row electrode is set at a high voltage,
particles in the cell move to the bottom surface of the cell. The
bottom of the cells will be covered by the white particles and the
color of the dielectric solvent (i.e., red, green or blue) is seen
through the top transparent conductor layer.
[0166] As illustrated in FIG. 12D, when the bottom electrode and
in-plane electrodes are both set at a high voltage, and the top row
electrode is set at a low voltage, particles in the cell move to
the top surface of the cell. When the white particles in the cell
migrate to the top of the cell, the resulting white color is seen
from the top and the pixel is in the white state.
[0167] As illustrated in FIG. 12E, when the top row electrode and
bottom column electrode are set at a high voltage, and the in-plane
electrodes are set at a low voltage, particles move to the area of
the small in-plane electrodes, resulting in the black background
color being seen through the top transparent conductor film.
[0168] This design has the advantage of reducing the up-down
switching time. As the bottom electrode is controlled by the TFTs,
the storage capacitor holds the voltage on the bottom electrode.
During the non-scan phase, the electrode with a voltage set by the
storage capacitor continues to generate the electric field and
control the movement of the particles. This effectively reduces the
cell switching time.
[0169] While the present invention has been described with
reference to the specific embodiments thereof, it should be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted without departing from the
true spirit and scope of the invention. In addition, many
modifications may be made to adapt a particular situation,
materials, compositions, processes, process step or steps, to the
objective, spirit and scope of the present invention. All such
modifications are intended to be within the scope of the claims
appended hereto.
[0170] It is therefore wished that this invention to be defined by
the scope of the appended claims as broadly as the prior art will
permit, and in view of the specification.
* * * * *